Obama plans to kick-off rallies Monday in states that historically have voted for Republican candidates. He is expected to discuss the weak U.S. economy and the difficulties of high unemployment and rising gas costs.

The 46-year-old has made history as the first African-American to represent a major political party in the race for the White House.

Obama's rival - 71-year-old McCain - is already airing television advertisements focusing on national security and his experience in the Vietnam War.

Both men also are working to find a vice presidential candidate to run with them in the November election.

McCain - who has been a lawmaker for more than 20 years - says he has the experience needed to lead the United States. Political analysts say McCain's prospects for the White House could be hurt by his affiliation with President George Bush, who is one of the most unpopular U.S. leaders in recent history.

Analysts say Obama faces the challenge of presenting a concrete strategy for change, not just speeches about change. They say he also could have difficulty drawing support from states where racism persists.

Democratic Party Senator Hillary Clinton - Obama's main rival - had hoped to become the first female to be nominated for president. She ended her campaign Saturday by declaring her full support for Obama. Clinton is the wife of former President Bill Clinton.

Obama says he is honored by Clinton's endorsement. He praised her for helping promote equal rights for men and women.